Explosive detection system

ABSTRACT

A system for detecting the presence of explosives contained in an object under observation, including a cavity structure for receiving the object and a radiation source for producing thermal neutrons directed to the object under observation. Gamma rays are produced to represent the presence of explosives and as an example the concentration of nitrogen contained in the object. Inorganic scintillators are located within the cavity structure to detect the gamma rays and produce an output signal representative of the presence and concentration of the nitrogen and/or other elements contained in the object. The inorganic scintillators are formed as a ring around the cavity structure to detect the nitrogen and/or other elements within at least one particular plane passing through the object. The object under observation is moved through the cavity structure to detect the nitrogen in successive planes to build up a three dimensional profile of explosives concentration.

This is a division of application Ser. No. 321,511 (now U.S. Pat. No. 5,006,299) filed Mar. 9, 1989, which in turn is a continuation of application Ser. No. 053,950 filed May 26, 1987 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an explosive detection system. Specifically, the present invention relates to a detection system using thermal neutrons in combination with a detector formed from an inorganic scintillator such as a scintillator made of sodium iodide to provide for a more efficient detection of explosives. A specific detection system may be provided using a ring of detectors to detect the presence of explosive within a particular plane of an object under inspection and with a continuous movement of the object providing for a three dimensional profile of the explosive.

2. Description of the Prior Art

A great need exists for the scanning of luggage, baggage and other parcels for the detection of any explosive material contained or concealed within their confines. For example, a large number close to two million (2,000,000) pieces of luggage are checked and/or carried onto aircraft daily by close to seven hundred and fifty thousand (750,000) passengers within six hundred (600) airports extending across the country. Many more packages move through the mails or are shipped to sensitive buildings. There is a possibility, albeit small, that any one piece of luggage or parcel may contain explosive material. It is, therefore, desirable to protect the public by providing detection systems to scan the luggage and parcels to detect the presence of any explosive material.

It thereby follows that any system of checking luggage or parcels must have a very high probability of detection in order to be effective. Because of the large number of parcels processed, a high throughput is necessary for practicality. In addition, because of the large number of scanned items, any detection system is bound to occasionally give a false alarm. The probability of these false alarms must be minimized in order to provide for an effective explosive detection system. This is true since when an alarm occurs it is not known, at that time, whether it is true or false. This means that each time an alarm occurs, a passenger or parcel must be detained for further investigation. If false alarms are significantly high the nuisance level and the delays could be unacceptable to the public. It is, therefore, important that any explosive detection system must have a very high probability of detection and a high throughput rate, and yet, at the same time, have a very low probability of false alarms. These conflicting criteria have hampered efforts in the past to build a reliable and usable system

In general, prior art systems have not met the desired characteristics of having a high probability of detection with a low probability of false alarms at acceptable throughput rates. As an example, one such prior art system is shown in U.S. Pat. No. 3,832,545. This patent provides for a system for the detection of nitrogen, which is generally present in the explosive materials to be detected. The object under observation is positioned within a cavity structure and the object is bombarded by thermal neutrons. The thermal neutrons interact with any nitrogen contained in the object to induce the emission of gamma rays at an energy level characteristic of the nitrogen element.

The emitted gamma rays are then detected by two parallel planar arrays of gamma ray detectors. U.S. Pat. No. 3,832,545 specifically provides for the use of liquid or plastic type organic scintillator detectors having an end surface for viewing a portion of the article being inspected and with the length of the organic scintillator being substantially greater than the effective diameter of the end surface. As described in this prior art patent, the array of organic scintillators provides for a crude two dimensional profile of the nitrogen content within the object being inspected. The two dimensional concentration profile of the nitrogen is then used to provide for a detection of an explosive. This type of prior art system has a number of deficiencies, including both a low gamma ray intensity and spatial resolution of the detection of the concentration of the nitrogen contained in the object under inspection, and the insensitivity of the system to detect explosive devices which are deliberately positioned within the object under inspection so as to defy detection. Because of the use of liquid or plastic type scintillators, long times are required to make a decision about any object. The system described in the prior art patent is also slow and cumbersome in operation which is a further limitation to its usefulness.

Other types of prior art explosive detection systems depend upon the prior seeding of explosive materials with a tracer material, such as a radioactive tracer. Although this type of system could be very useful if all explosive material were manufactured with such tracer material, because of the large amount of explosive material which has already been manufactured and because of the difficulty of controlling the manufacture of all explosive material so as to contain such tracer material, this type of system is not practical. A usable system must be able to detect the presence of explosive material of a conventional type and of an unconventional type, whether disposed within an object either in its original manufactured form, or if deployed within the object so as to attempt to confuse or evade the detection system. The prior art systems have not met these various criteria and cannot produce the desired high probability of detection with the relatively low production of false alarms.

An acceptable response to the explosive threat to aviation, mails, or shipping requires detection techniques that are highly sensitive, specific, rapid and non-intrusive. The efficient detection of nitrogen, at this point, offers the best overall solution. It is, therefore, important that this detection of nitrogen be provided to give the maximum information of the physical parameters of the explosive, such as density and spatial distribution The use of nuclear based techniques which subject the luggage or parcels to thermal neutrons can be the basis of a system to produce the desired results, but this system cannot be based on the prior art techniques. It is important that the intensity, energy and spatial distribution of the detected radiations from the object under observation must be provided in such a way so as to help to determine the presence or absence of explosives and this has not yet been accomplished.

In addition to high detection sensitivity and low false alarm the detection of the explosive should be independent of the specific configuration and must be non-intrusive in order to protect privacy. The detection equipment, of course, must be non-hazardous to the contents of the checked items and to the operating personnel and environment. Other more general criteria are that the system must be reliable, easily maintained and operable by relatively unskilled personnel and that the cost must be low enough so as to be non-burdensome to airlines and airports. Finally, it is desirable, when all other requirements achieved that the size of the system be relatively small so that the system may be useful in a wide variety of environments.

In addition to the nuclear based systems described above, non-nuclear systems have also been investigated. These systems have achieved relatively high efficiencies of detection, but generally have relatively high false alarm rates and have long screening times. These type of non-nuclear systems, therefore, by themselves cannot achieve the desired results. It is possible to combine a non-nuclear system with a nuclear system, but the present invention is directed to specific improvements in the nuclear based type of system.

In order to develop a proper explosive detection system, an understanding is required of the properties of the various explosives relevant to the specific techniques to be used. Although there are a large number of explosive types, a general classification into six major groups with minor variations, has been proposed. The proposed classification scheme includes the following types of explosives: (1) nitroglycerine based dynamites, (2) ammonium nitrate based dynamites, (3) military explosives, (4) homemade explosives, (5) low order powders, and (6) special purpose explosives.

Nitroglycerine based dynamites are the most common form of explosives. The basic composition includes equal amounts of nitroglycerine and ethylene glycol dinitrate, plus a desensitizing absorber in the form of cellulose in either sodium or ammonium nitrate.

The ammonium nitrate based dynamites have been replacing nitroglycerine based dynamites in popularity. These types of dynamites are commonly referred to as slurries or water gels. The two general types of ammonium based dynamites are the cap-sensitive and the cap-insensitive types. The former consists of aluminum, ammonium nitrate, ethylene glycol and water while the latter contains wax or fuel oil and water.

Military explosives are formed of Composition-4 (C-4), TNT and picric acid. C-4 is composed of cyclotrimethylene trinitramine (RDX) and a plasticizer.

Homemade explosives are diverse and are limited only by the creativity of the perpetrator. Ammonium nitrate (fertilizer) and fuel oil are the most common and available constituents.

Low order powders (black and smokeless) have typically been assembled in pipe bomb configurations and have been used extensively in that form. Black powder contains potassium nitrate, carbon and sulfur. Smokeless powder is primarily pure nitrocellulose or a mixture of nitrocellulose and nitroglyerine.

Special purpose explosives include detonating cords, blasting caps and primers. The explosive entities in the special purpose explosives are PETN, lead azide, lead styphanate, mercury fulminate and blasting gel.

In general, all of these explosives contain a relatively high amount of nitrogen ranging from nine to thirty five percent by weight. The nominal density of these explosives is typically 1.6 gm/cm³ and with ranges between 1.25 to 2 gm/cm³ or more. These physical properties demonstrate that the most unique signature of explosives is the high concentration and density of the nitrogen content. There are other physical factors that identify explosives, but these other factors do not form part of the present invention. However, one factor which is important is that most explosives have a minimum progagation thickness or diameter in order to be effective. The minimum propagation thickness entails a sizable contiguous body of explosives in the other two dimensions. This information is useful to the detection of explosives without making a specific assumption of the actual shape of the explosive.

In can be seen, therefore, that a nuclear detection technique can provide for the detection of the nitrogen content to reliably indicate the presence of a large nitrogen content. However, the frequent occurrence of nitrogen in non-explosive materials limits the level of detection sensitivity and merely detecting the presence or absence of nitrogen alone is not sufficient. Therefore, additional information is required beyond simply sensing the presence of the nitrogen. The present invention provides for this additional information using specific structures and a specific detection configuration to provide for a greater reliability in the detection of explosives.

SUMMARY OF THE PRESENT INVENTION

The basis for the explosive detection system of the present invention is the use of neutrons from a radioisotope or an electronic neutron generator which neutrons are then slowed down to create a cloud of low energy thermal neutrons within a cavity. The luggage or other parcels pass through the cavity and the thermal neutrons react with the variety of nuclei in the luggage or parcels and produce characteristic high energy gamma rays which may then be detected by external detectors. The detector processing electronics then converts the detected signals into pulses suitable for computer processing.

The present invention relates to the specific arrangement and type of the detectors relative to the use of thermal neutrons as a source to provide the proper information. The information may then be converted by computer processing to indicate the possible presence of an explosive threat. At a minimum, if there is a high enough count rate indicating the presence of a great deal of nitrogen, the system of the present invention can easily detect the presence of an explosive. The system of the present invention can also detect explosives provided in more unconventional configurations and at the same time reduce the number of false alarms to a relatively low level. The prior art detector systems in general provide for the gross detection, but cannot provide for the more sensitive detection of the unconventional configurations while, at the same time, providing for a relatively low level of false alarms.

The explosive detection system of the present invention includes the use of efficient inorganic scintillators capable of resolving closely spaced high energy gamma ray lines. Specifically, sodium iodide scintillators are used to provide for detection, but it is to be appreciated that other inorganic scintillators such as cesium iodide, bismuth germanate and barium floride scintillators may also be used. In addition, inorganic solid state detectors such as lithium-drifted germanium, high purity germanium or mercuric iodide may be used.

The inorganic scintillators of the present invention are arranged to form at least one ring of detectors so as to provide for a detection of a plurality of slices or parallel successive planes of the object under inspection as the object is moved continuously through the ring of detectors. In a specific embodiment of the invention, this ring is broken into sets of C-rings; and in order to provide for a better three dimensional representation, two spaced sets of C-ring detectors may be used with the open ends of the C-rings facing each other so as to provide for a detection completely around the object and with the plurality of successive planes building up a three dimensional profile of the object under inspection.

The system of the present invention is capable of scanning a continuous flow of luggage and parcels. In addition, the operation of the system may be fully automatic so that the system does not depend on operator experience or interpretation and thereby provides for an automatic detection of explosives.

BRIEF DESCRIPTION OF THE DRAWINGS

A clearer understanding of the present invention will be seen with reference to the following description and drawings wherein:

FIG. 1 illustrates a perspective view of a luggage and parcel inspection system;

FIG. 2 illustrates the system of FIG. 1 with a shield portion of the system removed;

FIG. 3 illustrates a detailed view of a conveyer path for the system showing the positioning of a pair of thermal neutron sources and sets of inorganic scintillator detectors constituting the C-ring detector array;

FIG. 4 illustrates a block diagram of the system showing the detection of particular gamma rays for detecting of explosive material and with waveforms (a), (b) and (c) representative of the signals at particular points in the system; and

FIG. 5(a), (b) and (c) illustrate typical spatial profiles of nitrogen concentration for explosive and non-explosive materials.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, an explosive detection system 10 includes a loading station 12 (which may consist of a scale to weigh the luggage) and an unloading station 14 (which may consist of a diverter to separate the alarmed luggage from the rest). The loading station leads to a continuous conveyer belt 16 which extends from the loading station 12 to the loading station 16 and has a continuous motion as indicated by the arrows 18. A central shield structure 20 encloses the explosive detection system and with two external wing portions 22 and 24 extending from the central structure 20 to enclose the conveyer belt 16 leading from and to the loading and unloading stations 12 and 14.

As can be seen in FIG. 2, wherein the shields 20, 22 and 24 are removed, the explosive detection system is positioned over a central portion of the conveyer belt 16. Specifically, the explosive detection system includes a cavity structure 26 through which the conveyer belt 16 passes. As shown in FIGS. 1 and 2, various articles of luggage and parcels may be positioned on the loading station 12 and may then be carried through the cavity 26 to the unloading station 14 by the conveyer belt 16.

The cavity 26 is formed by external wall members including a top wall 28, side walls 30 and 31 and a bottom wall (not shown) which is positioned below the conveyer belt 16. Extending through the wall members are thermal neutron sources, such as source 32, positioned at the top of the cavity and as shown in FIG. 3, neutron source 34 spaced from the neutron source 32 and positioned at the bottom of the cavity. Also as shown in FIGS. 2 and 3, detector structures are positioned to form two C-rings of detectors having their opened ends facing the neutron sources. This may be seen in FIG. 3 wherein the side detector structures 36 and 38 together with the bottom detector structure 40 are all associated with the neutron source 32. Similarly, side detector structures 42 and 44 together with the top detector structure 46 are all associated with the neutron source 34.

As shown in FIG. 3, the side detector structures are provided by two sets of four detectors located in each side detector structure 36 and 38. The bottom detector structure 40 includes two sets of seven detectors. The detectors associated with the neutron source 34 similarly include two sets of four detectors located in each side detector structure 42 and 44 and two sets of seven detectors located in the top detective structure 46. The detectors associated with the neutron source 32, therefore, form a C-ring of detectors having the opened portion of the C facing upward. In an opposite fashion the detectors associated with the neutron source 34 form a C-ring with the opened portion of the C facing downward. The combination of the two sets of C-ring detectors thereby provide for the detection of a complete ring around the object under inspection to produce a better three dimensional profile of the nitrogen distribution within any particular object passing through both sets of detectors.

It is to be appreciated that the invention is described with reference to the use of two C-ring detector structures and with each C-ring including two sets of parallel rows and columns of detectors. It should be appreciated that only a single C-ring structure may be used with only a single row and column of detectors. The use of the additional parallel sets of detectors improves the visualization of the profile of the concentration of nitrogen, but a simpler system could be used with a single C-ring and single rows and columns of detectors. It is also to be appreciated that more detectors could be used. A full ring of detectors could also be placed out of the plane of the source, around the cavity.

The actual structure of the neutron source and its environment, such as the structures 32 and 34, may be of any type. For example, the neutron source may be a radioisotope (such as 252 Cf) or an electronic neutron source (such as (D,D) or (D,T) generators). By collisions, mostly with the nuclei of the selected materials surrounding the source the neutrons are slowed down to create a cloud of low energy thermal neutrons within the cavity. The low energy thermal neutrons specifically interact with the variety of nuclei in the luggage or parcel. The interaction of the low energy thermal neutrons produces characteristic high energy gamma rays which are detected by the external rows and columns forming the C-ring detectors.

Each detector in the rows and columns preferably are formed of inorganic scintillators. Specifically, all of the detectors, such as represented by a detector 48, may be formed of an inorganic scintillator material, such as sodium iodide (NaI). Other inorganic materials may be used and as an example, inorganic materials such as cesium iodide (CsI), bismuth germanate (BGO-Bi₄ Ge₃ O₁₂) or barium fluoride (BaF₂) also may be used to provide for the detector structure. In addition inorganic solid state detectors such as lithium-drifted germanium (Ge(Li)), high purity germanium (HPGe) or mercuric iodide (HgI₂) may be used. The particular details of a specific detector structure do not form a part of the present invention, but the specific use of an inorganic scintillator with good energy resolution and efficiency to detect gamma rays produced by thermal neutrons provides for a unique detection of nitrogen and/or other elements to form a part of the present invention.

Although inorganic scintillators have been used in the past with thermal neutrons, this use was not for the detection of nitrogen in explosives, but rather to provide for the detection of chlorine, iron, chromium, etc. as a background component and not specifically for the detection of the nitrogen component and spatial distribution of the explosive material. Other uses of inorganic scintillators have been in combination with fast neutron sources so as to detect nitrogen, but this different type of neutron source provides for a different type of detection.

The present invention contemplates the specific combination of a thermalized neutron source with an inorganic scintillator, such as a sodium iodide detector. This specific combination provides for the capability of resolving closely spaced high energy gamma ray lines and specifically for detecting the particular gamma ray lines representative of the nitrogen content of explosives. These particular high energy gamma rays lines occur at 10.8 MeV. The inorganic scintillator detector is used because it is a very efficient detector and because it provides acceptable features in a number of areas. These areas include level of total count rate, the shape of the detector, availability of detector, reliability and cost. It is to be appreciated that the inorganic scintillator may also be used to detect other elements representative of an explosive.

As indicated above, the currently preferred inorganic material is sodium iodide, but other inorganic materials may be used. For example, bismuth germanate has a higher effective atomic number because of the bismuth and a higher density than the sodium iodide. The efficiency of a bismuth germanate scintillator is, therefore, higher than that of sodium iodide. However, bismuth germanate scintillators are inferior to sodium iodide in energy resolution and the cost for a bismuth germanate scintillator is much higher than that for sodium iodide and it also has a background component that can interfere with the nitrogen signal. However, both of these inorganic structures are superior to the organic scintillators used in the prior art devices.

For example, on the basis of the mean free path of 10 MeV gamma rays, sodium iodide and bismuth germanate are roughly 6 and 11 times more efficient than organic scintillators. Moreover, in terms of depositing the gamma energy in the scintillators, sodium iodide and bismuth germanate are roughly 10 and 40 times respectively more efficient than organic scintillators. The energy resolution, which is the ability to separate two lines, given as the peak's width at half the peak's height, is around 200 to 300 KeV for sodium iodide and 400 to 500 KeV for bismuth germanate at high energies and with the range depending on the crystal size and quality.

The main advantage of the prior art organic scintillators, which may be plastic or liquid, is their very fast time response permitting exceedingly high count rates. Because of the very high count rates, a high background from other neutron reactions can be handled easily and thereby eliminate the need for sophisticated cavity design. Another advantage of the organic scintillators is their relatively low cost and ease of manufacture. Even with these advantages with the use of organic scintillators, the use of the inorganic scintillators of the present invention, and specifically in the particular C-ring configuration, provides for a higher resolution and thereby a more efficient detection of any explosive material. The organic scintillators are inefficient detectors for high energy gamma rays and their gamma spectroscopical qualities are poor. Organic scintillators thereby have poor energy resolution and make the separation between nitrogen and deleterious signals, such as occur with C1, Fe, Cr or Ni, very difficult.

As can be seen in FIG. 3, any item to be scanned, such as a piece of luggage, passes through the cavity on the conveyor 16 and is subjected to the thermal neutrons produced by the thermal neutron source 32. At successive positions of the piece of luggage, the individual detectors 48, forming the row 40 and columns 36 and 38, provide for a cross sectional profile of any material containing nitrogen. The C-ring of detectors thereby provides for a two dimensional slice or plane of the nitrogen concentration and with a three dimensional profile built up by the successive slices produced as the luggage moves through the C-ring of detectors.

The two dimensional plane provided by the detector structures 36, 38 and 40 has less resolution at the upper end since the C-ring is not complete. Although a detector structure could also be provided along the upper surface of the cavity such detector structure could interfere with the production of thermal neutrons by the source 32 of such neutrons. A more efficient way of completing the ring is to have a second C-shaped group of detector structures provided downstream of the first group so that the luggage moves from the first C-ring of detector structures to the second C-ring of detector structures, the open ends of the C-rings in the first and second sets being opposite to each other. The information from the two sets of C-rings of detector structures may be merged electronically in a computer to provide for a complete picture. As indicated above, this picture forms a three dimensional image of the container such as the luggage and its contents by building up the successive slices or planes of information.

FIG. 4 illustrates in general the detection of the information by any one of the individual detectors 48. As shown in FIG. 4, neutrons from the sources, either 32 or 34, are thermalized and impinge on a piece of luggage or parcel as represented by the block 50. Each of the individual detectors 48 forming the C-ring detector structures detects the production of gamma rays. The reaction between the thermal neutrons and the nitrogen in the explosive or other material is as follows: ##STR1##

The first factor in the above equation is the nitrogen in the explosive or other material within the luggage. For example, wool, cotton, etc. all contain nitrogen. The nitrogen when bombarded with thermal neutrons, as shown by the second factor, produces nitrogen in a changed form (another isotope of nitrogen) plus gamma rays, of which approximately 14% are at 10.8 MeV. Each gamma ray as detected by a detector 48 produces an output from the detector as shown in waveform (a) in FIG. 4. As can be seen, the detector 48 produces an output signal having a height "h" and decaying exponentially to zero value at approximately one micro second. The detectors 48 are supplied with a high voltage from a high voltage source 52. The height "h" and the area under the decaying signal are both proportional to the gamma ray energy.

The output from each detector 48 is passed directly or through a preamp and amplifier 54 to produce an output signal as shown in waveform (b) in FIG. 4. It can be seen that the individual gamma ray is converted from the exponentially decreasing signal to a pulse signal having a height "H" which is proportional to the area under the signal shown in waveform (a). It is to be appreciated that each gamma ray received by each detector 48 produces successive signals representing the concentration of nitrogen.

The output from the preamp/amplifier 54 is passed through an analog to digital (A to D) converter 56 to produce a digital number representing the height "H" of the waveform (b) g of FIG. 4. It can be seen, therefore, that the outputs from the A to D converters 56 are a series of digital numbers representing the detection of gamma rays indicative of the concentration of nitrogen. A small range of the digital numbers corresponds to the gamma rays of interest. As more and more gamma rays are detected at each detector, the digital number from the A to D converters 56 at each point in time is counted. The counts of each digital number which occurs, which is proportional to the number of nitrogen gamma rays incident on the detector, are then coupled into a computer 58 for computation of a profile for each slice or plane of the object under observation and for the production of a three dimensional representation of the concentration of nitrogen of the object. Waveform (c) illustrates the profile of the spectrum received by the detectors 48 and with the space 60 between the two lines representing the area of interest, more specifically the gamma rays representing nitrogen.

FIGS. 5(a), (b) and (c) illustrate typical profiles for explosive material in a block form, non-explosive materials, such as a wool coat or jacket, and explosive material in sheet form. As can be seen in FIG. 5(a), which represents the detection from one column of detectors at four successive planes as the object moves past the dectectors, the high readings of 12 in two successive planes and 8 in a third successive plane represent a high concentration of nitrogen rich material probably representive of a block of explosive material. The detectors in the other column and along the row would confirm the presence of such block material. The large difference between readings in the profile of FIG. 5(a) show an unusual density of nitrogen material not typical in other types of items which contain nitrogen.

For example, FIG. 5(b) illustrates an item such as a wool coat or suit which may contain a relatively high amount of nitrogen, but with the nitrogen spread out in a diffuse pattern which would not be representative of an explosive material. Although the overall nitrogen content of the wool article is quite high, the concentration does not reach the levels of explosive material.

FIG. 5(c) illustrates an explosive material in a sheet form along one side or edge of the luggage and again, the concentration of nitrogen and high readings relative to the lower readings indicates the presence of something having a relative high concentration of nitrogen, plus a relatively high density for this concentration. This again would typically be a profile of an explosive material. The computer 58, therefore, may be programmed to identify such specific profiles and provide for an alarm such as through an alarm 62 so that the luggage or parcel may be subjected to a more thorough inspection.

The present invention is, therefore, directed to an explosive detection system using thermalized neutrons from a source to impinge on an object potentially containing explosive material and with the thermal neutrons reacting with the nitrogen contained in the object to produce gamma rays. The gamma rays are detected by inorganic scintillators and, in a preferred embodiment, the scintillators are arranged in a ring configuration to provide for a detection of a two dimensional slice or plane of the object under observation. The object is moved continuously through the ring of detectors so that successive slices or planes provide for the build up of a three dimensional profile of the nitrogen bearing material within the object under observation. The three dimensional profile may then be used to provide for a determination of the concentration and distribution of the nitrogen bearing material to make a determination whether such nitrogen bearing material has a profile likely to be an explosive material, such as high nitrogen density.

In a preferred embodiment, the inorganic scintillator is sodium iodide and with two oppositely disposed C-ring detectors having their open ends facing each other to provide for a complete profile of each slice or plane along all four sides. In addition, the detectors may be formed of sets of detectors in rows and columns to increase the detection capability by receiving additional gamma rays produced by the interaction of the thermal neutrons and nitrogen in the cavity. The present invention, therefore, provides for a greater resolution and efficiency in the detection of potentially explosive material and with this accomplished in a fast period of time so as to provide for an adequate throughput of the luggage or parcels through the detection system.

Although the invention has been described with reference to a particular embodiment, it is to be appreciated that various adaptations and modifications may be made and the invention is only to be limited by the appended claims. 

We claim:
 1. A method of producing a three dimensional profile of the concentration of a particular element in a particular object, including the following steps:providing a cavity structure for receiving the particular object, providing a source for producing thermal neutrons, directing the thermal neutrons within the cavity structure to the object to interact with the particular element in the object to form gamma rays of a particular energy, providing a plurality of detectors around the periphery of the cavity structure in a common plane with the thermal neutron source in the cavity structure and in a C-shaped ring configuration and in a closed loop with the thermal neutron source to detect the concentration of the particular element within the common plane, operating each of the detectors in said plurality of detectors independently of one another, moving the object through the cavity structure in a direction transverse to the common plane to obtain the detection by the detectors of the presence of the particular element in successive planes of the object parallel to the common plane, processing the detections by the detectors of the particular element in the successive planes of the object parallel to the common plane to obtain the three dimensional profile of the concentration of the particular element within the object.
 2. The method of claim 1 additionally including producing an alarm in response to the production of particular three dimensional profiles of the particular element in the object.
 3. The method of claim 1 additionally including the detectors being composed of inorganic material.
 4. The method of claim 3 wherein the inorganic detectors are provided from a group of materials including: Sodium Iodide (NaI), Cesium Iodide (CsI), Bismuth Germanate (BGO-Bi₄ Ge₃ O₁₂) and Barium Floride (BaF₂), and solid-state detectors including Lithium-drifted Germanium (GE(Li)), High Purity Germanium (HPGe), and Mercuric Iodine (HgI₂).
 5. The method of claim 4 wherein the inorganic detectors provided are formed from Sodium Iodide (NaI).
 6. The method of claim 1 wherein the C-shaped ring defining the disposition of the detectors has an open end and the thermal neutron source is disposed in the open end of the C-shaped ring.
 7. The method of claim 1 wherein the detectors are formed into a spaced pair of C-shaped rings each having an open end facing the open end in the other C-shaped ring and wherein the two C-shaped rings are spaced from each other in the direction of movement of the object and the two C-shaped rings are parallel to each other.
 8. The method of claim 7 wherein each of the C-shaped rings is formed of parallel sets of individual detectors and the detections of the particular element by the detectors of the parallel sets of individual detectors are processed to obtain the three dimensional profile of the concentration of the particular element within the object.
 9. A method of producing a three dimensional profile of the concentration of a particular element in a particular object, including the following steps:providing a cavity structure for receiving the particular object, producing thermal neutrons from a first source within the cavity structure from a stationary source to interact with the object within the cavity structure in forming gamma rays of a particular energy, providing for a movement of the object through the cavity structure in a particular direction, disposing a plurality of detectors within the cavity structure in a common plane with the thermal neutron source so as to enclose the object moving therethrough, said common plane being transverse to the particular direction of movement of the object through the cavity structure, independently producing output signals from each of the detectors representative of the gamma rays detected by such detector in each successive one of the transverse planes in the object as a result of the movement of the object in the particular direction in the cavity structure, and processing the output signals from the detectors at progressive instants of time to obtain an indication of the three dimensional profile of the particular element in the object.
 10. A method as set forth in claim 9, including the step ofproducing an alarm when the indication of the three dimensional profile of the particular element in the object represents a particular concentration of the particular element in the object.
 11. A method as set forth in either of claims 9 or 10 whereinthe transverse plane is perpendicular to the particular direction.
 12. A method as set forth in either of claims 9 or 10 whereinthe detectors have an open configuration at one end of the transverse plane and wherein the detectors constitute a first plurality and the thermal neutron source constitutes a first thermal neutron source and wherein thermal neutrons are produced within the cavity structure from a second source displaced in the particular direction from the first source and wherein a second plurality of detectors are disposed within the cavity structure with the second source in a plane parallel to the transverse plane for an independent production by each of such detectors of output signals representative of the gamma rays detected by such detector in each successive one of the transverse planes in the object as a result of the movement of the object in the particular direction in the cavity structure and wherein the detectors in the second plurality have an open configuration int he plane defined by such detectors and the second neutron source at the end opposite the open end defined by the detectors in the first plurality and wherein the output signals from the detectors in the first and second pluralities are processed at the progressive instants of time to obtain the indication of the three dimensional profile of the particular element in the object.
 13. A method as set forth in either of claims 9 or 10 whereinthe thermal neutron source and the detectors are disposed in the cavity structure around the periphery of the cavity structure and wherein the detectors are disposed in a C-ring configuration having an open end and the thermal neutron source is disposed in the open end of the C-ring configuration.
 14. A method as set forth in claim 12 whereinthe first thermal neutron source and the detectors in the first plurality are disposed in the cavity structure around the periphery of the cavity structure and wherein the detectors in the first plurality are disposed in a first C-ring configuration having an open end and the first thermal neutron source is disposed in the open end of the first C-ring configuration and wherein the second thermal neutron source and the detectors in the second plurality are disposed in the cavity structure around the periphery of the cavity structure and wherein the detectors in the second plurality are disposed in a second C-ring configuration having an open end opposite the open end in the first C-ring configuration and wherein the second thermal neutron source is disposed in the open end of the second C-ring configuration.
 15. A method as set forth in either of claims 9 or 10 whereinthe detectors are inorganic. 